Fuel cells are regarded by many as a promising source of power for a wide array of devices, including vehicles, as well as a host of other portable and stationary applications. Fuel cells are capable of providing high energy efficiency and relatively rapid start-up. Moreover, fuel cells are capable of generating power without generating the types of environmental pollution that characterize many other sources of power. Thus, fuel cells can be a key to meeting critical energy needs while also mitigating environmental pollution by substituting for conventional power sources.
Notwithstanding the advantages afforded by increased utilization of fuel cells, their wide-spread commercialization is likely to hinge on whether and the extent to which the cost per unit power associated with fuel cells can be reduced including the precious metal cost. For transportation applications, U.S. Department of Energy (DOE) has set a 2015 technical target for the electrocatalysts which is generating a rated power of 1 W/cm2 with a total Pt loading of 0.2 mg/cm2 resulting in a Pt utilization of 0.2 gPt/kg. U.S. Dept. of Energy, Hydrogen, Fuel Cell & Infrastructure Technologies Program Multi-Year Research, Development and Demonstration Plan (2007). This level of utilization will have substantial benefits, including a substantial cost reduction due to reduced amounts of platinum (Pt) required for the same or improved fuel cell output. Indeed, a particularly promising avenue for commercialization is to improve Pt utilization while also optimizing electrode structure so as to achieve a high Pt specific power density.
One obstacle to achieving this aim, however, is the fact that conventional catalyst supporting materials, such as carbon black Vulcan XC-72R, have numerous micropores in which Pt nanoparticles can become trapped. This typically results in a failure in establishing the three-phase boundary (TPB) among gas, electrolytes, and the electrocatalyst of a fuel cell. The corresponding fraction of Pt is therefore not utilized since the electrochemical reactions cannot occur at these sites, thus causing a reduction in the level of Pt utilization. Moreover, carbon black can be corroded under the severe conditions inherent in the cathode of the fuel cell, resulting in low cell stability and reduced service life.
More recently, carbon nanotubes and nanofibers have been examined as possible catalyst supports in proton exchange membrane fuel cells (PEMFCs) because carbon nanomaterials typically exhibit high conductivity and large specific surface areas. Additionally, such carbon nanomaterials possess relatively low microporosity and typically exhibit excellent resistance to electrochemical corrosion.
A conventional processes for fabricating carbon nanotube-based and carbon nanofiber-based catalyst layers for use in a PEMFC is to disperse carbon nanotubes (CNTs) or carbon nanofibers (CNFs) in a binder, such as Teflon or Nafion, to form a slurry that is then used to coat the gas diffusion layer. A significant problem inherent in the conventional process, however, is that the addition of the binder during the fabrication stage tends to isolate carbon nanotubes in the electrocatalyst layer, leading to poor electron transport and degradation or elimination of the Pt active surface.